Dye-sensitized solar cells are solar cells in which a dye that absorbs sunlight releases electrons upon absorbing light, thereby generating electricity. An article published in 1991 by Michael Gratzel of the Ecole Polytechnique Federale de Lausanne (EPFL) in Switzerland initiated the research into these solar cells. In terms of the cell mechanism, when light enters the cell, the dye inside the cell enters an excited state, thereby releasing electrons. These electrons are transported through titanium oxide (TiO2) to a transparent electrode, and then flow out of the cell. Meanwhile, the dye that has been converted to cations as a result of the electron release receives electrons that have been supplied from the other electrode via iodine (I) within the electrolyte, and is thereby returned to its original state.
Examples of the types of properties required for the transparent electrode used in these types of solar cells include low resistance, favorable thermal stability and chemical stability, high light transmittance, good moisture resistance, and low cost. As the transparent conductive film of an electrode that satisfies these properties, a fluorine-doped tin oxide film (FTO film) is typically preferred to the more tin-doped indium oxide film (ITO film), as it exhibits superior stability under both thermal and chemical conditions.
However, compared with ITO films, FTO films exhibit inferior transparency and conductivity, and therefore films have been investigated in which an FTO film is laminated on top of an ITO film, thereby improving properties such as the transparency in addition to the properties of thermal stability and chemical stability (for example, see Patent Document 1).